The present application relates to industrial equipment and more particularly to mechanical/hydraulic presses.
Almost every existing machine and much equipment in everyday use, e.g. refrigerators, heating systems, automobiles, airplanes, office furniture, contains metal parts produced by mechanical or hydraulic presses. These presses stamp, draw, or extrude metal blanks or billets to produce the parts desired. Almost all mechanical presses utilize a slider-crank linkage to convert crank rotation to press slide displacement. FIG. 1 shows a schematic representation of such linkages. OA is the crank, AB is the slider.
Slider-crank presses have high stroke rates enabling production of stamped and shallow drawn parts at high rates per minute. These machines, however, generate their rated force over a short distance, only. As an example, a 1,000-ton press 20 ft high, possessing a 20 in total stroke, produces its rated force over only the last inch of its stroke. If the slide is loaded above one inch from bottom dead center (BDC), required crank torque and side load on slide gibbing and frame increases greatly. Slide load must be significantly reduced to avoid overloading the press drive and generation of high loads on slide bearings and gibs.
A 1,000-ton hydraulic press possessing the same total stroke and force rating as a mechanical press easily can exert its rated force over six inches enabling production of metal parts by deep drawing or extrusion in addition to executing stamping and shallow drawing operations. The accompanying disadvantages are low stroke rate, only a few parts can be produced per minute, low mechanical efficiency, greater complexity, and higher maintenance cost.
Referring to FIG. 2, a schematic representation of a press linkage is shown in which two links are added to the slider-crank linkage. Link 1 is the crank, link 2 is the drag link, link 3 is the lazy link, oscillating during crank rotation, and link 4 is the slider. This mechanism is termed a four-bar linkage. When the links are properly sized, rated press force is generated over a long stroke, similar to that of a hydraulic press, while maintaining a much higher stroke rate than can be attained by a hydraulic press. This capability makes it possible to replace several hydraulic presses with a single four bar press. A second advantage of four-bar press use is tooling shock reduction in stamping operations. The slide velocity of a four-bar stamping press as the slide enters its work stroke, operating at the same production rate as a slider-crank press, is much lower than that of slider-crank press, reducing kinetic energy at impact by over 80% with concomitant reduction in noise, improving the environmental quality of the workplace. Currently, the noise level in a room containing 20 35-ton slider-crank stamping presses prevents conversation. Ear protection is mandatory; information is transmitted by hand signals. The reduction in impact energy also enables use of high wear-resistant steel punches which have only nominal shock resistance in place of tough, relatively ductile tool steel punches which have lower tool life, markedly reducing tooling cost.
Presently, four bar presses have a stroke rate limitation resulting from sudden changes in slide acceleration or “jerk”. The slow movement of the slide over a relatively long work stroke must be compensated for by rapid slide return if the production rate achieved by a slider-crank press of similar tonnage is to be matched. This rapid change causes a rapid change in slide acceleration which, in turn, causes linkage pin-bearing contact areas to suddenly shift and generate “linkage slam”. Linkage slam is directly proportional to the third power of crank rotation or stroke rate, and consequently suddenly occurs as stroke rate is increased. This cannot be tolerated since linkage bearing pound-out and failure will occur shortly thereafter if the stroke rate is not reduced.
Use of larger bearings and pins to reduce bearing stress and minimize pound-out enables attainment of higher stroke rates but substantially increases four-bar press cost because the increased size of the links requires use of a relatively large press crown and frame. Generally, it is preferable to use smaller links and limit stroke rate to a value at which linkage slam does not occur.
Four-bar linkages also generate a shaking force, developed by the skewed elliptical movement of the center of gravity of the drag link and the crank during a press stroke, limiting stroke rate. This force is reduced by link design minimizing element inertia enabling use of higher stroke rates.
Accordingly, there is a need for presses and other industrial equipment that are not limited as such.